1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a secondary battery in which the fixation of a lead plate to the top of a cap plate is enhanced and connection terminals are not exposed to the outside during hot-melt molding.
2. Discussion of Related Art
Chargeable/dischargeable batteries (also know as secondary batteries) are widely used in high-technology electronic apparatuses such as cellular phones, notebook computers, and camcorders. In particular, lithium secondary batteries have high energy density per unit weight and are widely used as power sources of electronic apparatuses.
Conventional polygon type lithium secondary batteries include a polygonal can, an electrode assembly accommodated in the can, a cap assembly that closes the opening of the can and that includes a cap plate, a lead plate and a breaker that are coupled with the upper surface of the cap plate, a protective circuit unit connected to the lead plate and the breaker through connection terminals, and a molding resin part with which a gap between the cap assembly and the protective circuit unit is filled. However, the structure and position of the conventional lead plate creates problems for the connection terminals.
Referring to FIGS. 1 and 2, an electrolyte injection hole is formed on one side of the cap plate 110 and a sealing unit 160 seals the electrolyte injection hole after the electrolyte is injected. The sealing unit 160 may have various shapes. For example, after a ball having a diameter larger than the diameter of the electrolyte injection hole is put on the entrance of the electrolyte injection hole and is mechanically press fitted into the electrolyte injection hole to form the sealing unit 160. Welding is then performed along the edge of the sealing unit 160 to seal the electrolyte injection hole. The ball is commonly formed of Al or a metal including Al, the ball material being the same material as the cap plate 110 in order to facilitate welding. In another method, after a thin sealing plate larger than the electrolyte injection hole is brought into surface contact with the top of the electrolyte injection hole, welding is performed along the edge of the sealing plate to seal the electrolyte injection hole.
Typically, the sealing unit 160 of the electrolyte injection hole and the welding part are not even but slightly protrude above the upper surface of the cap plate 110. Also, in order to prevent the electrolyte from leaking, the sealing unit 160 and the welding part are commonly coated with a hardener such as an ultraviolet (UV) hardener so that the sealing unit 160 and the welding part protrude above by the thickness of the coating film.
As illustrated in FIG. 1, although a through hole is formed on the bottom plate of a conventional lead plate 410, since the lower ends of the both side walls of the lead plate cross the sealing unit 160 and the welding part or the hardener coated part that protrude above the cap plate 110, the lead plate 410 is not firmly welded to the cap plate 110.
FIG. 2 is a front view illustrating that the lead plate 410 and the protective circuit unit 300 of the secondary battery illustrated in FIG. 1 are connected to each other through the connection terminal 360. The shapes of the side walls of the lead plate 410 and the position in which the connection terminal 360 is connected to the protective circuit are limited to some extent due to the protruding parts of elements in the protective circuit unit 300. The conventional connection terminal 360 is commonly L-shaped in a front view so that a part of the L-shaped part is not welded to the lead plate 410.
Therefore, when a part of the connection terminal 360 is welded to the side wall of the lead plate 410 and the connection terminal 360 is bent to be connected to the protective circuit, the part of the connection terminal 360 that is not welded to the lead plate 410 and that is not supported may protrude or may be deformed so that the connection terminal 360 may be exposed to the outside during hot-melt molding.